Final answer:
Upon opening a valve, water begins to flow through it, consistent with the continuity equation, which states that the product of cross-sectional area and velocity of a fluid is constant. The pressure local to the valve might change, but this does not necessarily affect overall water pressure in the system, and is also subject to Bernoulli's principle.
Step-by-step explanation:
When a valve is opened, water flows through the diffuser. According to the continuity equation, for an incompressible fluid like water, the product of the cross-sectional area (A) and the velocity (V) at any point in a streamline is constant (i.e., A1V1 = A2V2). Hence, if the cross-sectional area of the pipe changes, the velocity of the water will adjust so that the product of area and velocity remains constant. Opening a valve typically increases the available cross-sectional area for the water to flow through, so the water flows out of the opened valve.
This action does not necessarily result in a decrease of overall water pressure in the system; instead, the pressure locally changes due to the continuity and conservation of energy. This is exemplified by observing that water speed increases as it flows from a wider section to a narrower section of a pipe, which is accompanied by a pressure drop according to Bernoulli's principle. Therefore, in the case of a valve being opened, the water should flow through the opened section due to the pressure difference, unless the system has some other restriction or pump failure happening concurrently.